This invention relates generally to an onboard system for monitoring vehicular emissions and more particularly to a diagnostic system for monitoring such emissions.
Government regulations require vehicles equipped with internal combustion engines to have emission monitoring systems conventionally known as OBD (On-board Diagnostic Systems) to advise the operator of the vehicle when the gaseous pollutants or emissions produced by such vehicles exceed government regulatory standards. Government regulatory standards set emission threshold levels which the vehicle cannot exceed when operated pursuant to a specified driving cycle such as that set forth in a FTP (Federal Test Procedure). The FTP requires the vehicle be operated at various acceleration/deceleration modes as well as at steady state or constant velocity at various specified speeds.
One of the principal components of the vehicle""s emission system is the catalytic converter, typically a TWC (Three Way Catalystxe2x80x94NOx, hydrocarbons and oxides, i.e., CO). TWCs store oxygen when the engine operates lean and release stored oxygen when the engine operates rich to combust gaseous pollutants such as hydrocarbons or carbon monoxide. As the catalyst ages, its ability to store oxygen diminishes and thus the efficiency of the catalytic converter decreases.
In order to determine the efficiency of the catalytic converter, systems monitor the ability of TWCs to store oxygen to determine failure of the catalyst. Typically an EGO (exhaust gas oxygen sensor) is placed upstream of the TWC and an oxygen sensor is placed either within or downstream of the TWC to sense the oxygen content in the exhaust gas. The signals are adjusted for the time it takes the exhaust gas to travel from the precatalytic converter sensor to the postcatalytic converter oxygen sensor. The adjusted signals are then compared to ascertain the storage capacity of the TWC when the engine is in a lean or stoichiometric mode.
The principal disadvantage of this method is simply that the oxygen storage capacity of the TWC has been demonstrated to poorly correlate with hydrocarbon conversion efficiencies. See J. S. Hepburn and H. S. Gandhi xe2x80x9cThe Relationship Between Catalyst Hydrocarbon Conversion Efficiency and Oxygen Storage Capacityxe2x80x9d, SAE paper 920831, 1992 and G. B. Fischer, J. R. Theis, M. V. Casarella and S. T. Mahan xe2x80x9cThe Roll of Ceria in Automotive Exhaust Catalysis and OBD-II Catalyst Monitoringxe2x80x9d, SAE paper 931034, 1993. Another significant disadvantage of current monitoring systems is that for such methods to be applied to low emission vehicles and ultra-low emission vehicles, it will be necessary to monitor increasingly smaller portions of the TWC leading to less reliable correlations to total TWC performance. Finally, any system which attempts to evaluate the efficiency of the catalytic converter by ascertaining the gas composition of an exhaust stream before and after the TWC is inherently flawed because i) the speed of the gas has to be precisely determined even though the gas stream passes through a tortuous path within the converter conducive to producing uneven flow for various gas stream slips and ii) the gaseous reactions within the TWC are fundamentally kinetic in nature and vary in a complex manner depending on the speed and particular composition of the exhaust gas at any given instant.
Vehicles are equipped with engine microprocessors or ECMs (engine control modules) that are sophisticated, high powered devices capable of processing input from any number of sensors depicting operating conditions of the vehicle and rapidly issuing engine control signals in response thereto. An ECM can be programmed to perform on-board monitoring of the emissions system. U.S. Pat. No. 5,490,064 to Minowa illustrates such a control unit which includes in its functions an on-board self diagnostic emissions monitoring process using conventional pre and postcatalytic O.sub.2 sensors, digital filtering and exhaust gas speed to correlate the sensor readings to one another to determine failure of the catalytic converter. U.S. Pat. No. 5,431,011 to Casarella et al. likewise illustrates precatalytic and postcatalytic converter O.sub.2 sensors whose signals are processed by the CPU in the ECM along with other vehicle operational signals. In Casarella, a two-stage analyzing technique is utilized. Filtered signals are collected in a first stage and analyzed. If the first stage analysis indicates a failure, then a more thorough or rigorous second stage scrutiny of a number of signals which can affect performance of the catalytic converter is conducted before indicating failure of the converter. Despite the sophistication employed in the computer program and the ECM, the aforementioned system is inherently flawed because of the defects in the sampling system discussed above.
Catalysts are commonly used as part of exhaust systems to treat motor vehicle exhaust in order to minimize air pollution. The reduction of pollution from motor vehicles is mandated by the Environmental Protection Agency through the Environmental Protection Act. As part of the process to assure compliance, it is common for various motor vehicle regulatory bodies to mandate tailpipe testing of automobiles on a regular basis. In order to avoid the expense of such emissions, inspection and to assure that automobiles on the road are complying with the environmental laws and regulations, there are efforts to develop a system which can sense when the exhaust system is not compliant and signal the vehicle operator accordingly.
It is the goal to monitor the exhaust gas of a motor vehicle during normal operation to determine whether the catalytic converter is performing as required. The apparatus and method to accomplish this is commonly referred to as on-board diagnostics (OBD). The strategy which is contemplated is that the performance. of the catalyst is determined based on sensing the exhaust gases to determine whether the catalyst is performing as specified and required. Different sensing means have been proposed but all are required to signal the motor vehicle operator if the catalyst is failing to operate as required.
Sensors useful to measure various components in gaseous exhaust streams such as motor vehicle exhaust streams are known. Useful sensors include oxygen sensors and NOx sensor assemblies. Such oxygen sensors include on/off sensors known as heated exhaust gas oxygen sensors (HEGO) and universal exhaust gas oxygen sensors (UEGO) which is an on/off sensor plus a linear signal which is a function of the air to fuel ratio. Various oxygen sensors have been used and are disclosed in the art including the above referenced sensors.
Other approaches to sense whether the catalyst is performing include the use of dual oxygen sensors. In accordance with this method, one oxygen sensor is located upstream of the catalyst and the other downstream of the catalyst. The signals from the upstream and downstream sensors are compared and correlated to the emissions, typically hydrocarbon emissions, to determine whether the catalyst is functioning to reduce hydrocarbon emissions to achieve compliance with the regulations. If the amount of emissions is calculated to exceed a specific amount, a signal can be sent to the motor vehicle console to alert the operator that the system to treat exhaust has failed and repair is required.
The use of a dual oxygen sensor system has been reported in SAE Technical Paper Series No. 900062, Clemmens, et al., xe2x80x9cDetection of Catalyst Performance Loss Using On-Board Diagnosticsxe2x80x9d, presented at the International Congress and Exposition, Detroit, Mich., Feb. 26-Mar. 2, 1990. This paper reviews the history of such systems which are commonly referred to as On-Board Diagnostic Systems (OBD). This early study was indicated to be a proof of concept testing study to identify serious losses in catalyst efficiency with a dual oxygen sensor method. In accordance with this disclosure, testing was conducted at steady state conditions. The results showed that this approach resulted in measurable differences in the pre- and post-oxygen sensor signals between catalyst with good and poor conversion efficiencies.
SAE Technical Paper Series No. 910561, Koupal, et al., xe2x80x9cDetection of Catalyst Failure On-Vehicle Using the Dual Oxygen Sensor Methodxe2x80x9d was presented at the International Congress and Exposition, Detroit, Mich., Feb. 25-Mar. 1, 1991. This paper presents the results of a test program that used a dual oxygen sensor coupled with a simulated On-Board Diagnostic Algorithm to attempt detection of seriously deteriorated catalytic converters on a test vehicle operated over the Federal Test Procedure (FTP). Previous work is reviewed which relates to determining the methodologies to detect catalyst failure by observing the effects of three-way catalyst (TWC) conversion activity on a response pattern generated between an oxygen sensor. place upstream and oxygen sensor place downstream of the catalyst. One analytical method is referred to which quantified the fluctuation and sensor response by calculating the area underneath the sensor curves for a discrete time period, then taking the difference in sensor wave form area values. This integrated area difference method has been reported in the above reference to Clemmens. The background further references a study in 1980 by A. H. Meitzler, xe2x80x9cApplication of Exhaust-Gas-Oxygen Sensors to the Study of Storage Effects in Automotive Three-Way Catalystxe2x80x9d, SAE Technical Series No. 800019 which used the response delay of a downstream oxygen sensor to an instantaneous air/fuel shift as an indicator of a catalyst oxygen storage mechanism. Koupal studied the adaption of the integrated area difference algorithm developed by Clemmens to on-vehicle test results. The results were that the dual oxygen sensor method using integrated area difference analysis, was able to distinguish between good and bad catalyst under controlled conditions.
Presently, the method disclosed in this SAE Technical Paper Series 910561 is a method of using dual oxygen sensors for on-board diagnostic system measurement. However, it is extremely difficult to obtain consistent in-field measurements due to inconsistencies and, often, insufficient air to fuel swings.
In thesis, xe2x80x9cAn Engine Test to Measure the Oxygen Storage Capacity of a Catalyst (provide source), the oxygen storage capacity of a catalyst is assessed to determine its efficacy based on measurements of upstream and downstream air to fuel ratio.
One method to determine whether the catalyst is performing is to measure the switch ratio of the downstream versus, the upstream EGO sensors to determine the oxygen storage capacity of a catalytic converter. Based on this measurement the performance can be assessed (J. W. Koupal, M. A. Sabourin, W. B. Clemmens, xe2x80x9cDetection of Catalyst Failure On-Vehicle Using the Dual Oxygen Sensor Methodxe2x80x9d, SAE 910561, 1991). Currently, this is the most common method of using dual oxygen sensors for on-board diagnostic system measurements. However, it is extremely difficult to obtain consistent in-field measurements due to inconsistent and often insufficient air to fuel swings.
Another method of using dual oxygen sensors includes biasing the engine air to fuel ratio either rich or lean, and then determining the time it takes for the downstream HEGO sensor to sensor switch in the engine operating condition, vis-a-vis the upstream HEGO sensor. While this is a more reliable method of determining the oxygen storage capacity of the catalytic converter, it is intrusive. The measurement procedure involves changing the operation condition of the vehicle.
There are two inherent problems relating to the use of the oxygen sensors for on-board diagnostic measurements. One, is that there is no strong relationship between the oxygen storage in the catalytic converter and the hydrocarbon conversion performance of the catalytic converter. Secondly, it is difficult to determine a mode of operation of the vehicle under which reliable, meaningful comparisons can be made between the two sensors. It is therefore a continuing goal to devise an algorithm in combination with a sensor strategy to diagnose working of the catalytic converter.
An on-board catalyst diagnostic for monitoring the conversion efficiencies by measuring oxygen storage capacity (OSC) of the converter.
Using the OSC as a signature to monitor the catalyst conversion efficiency by directly monitoring the OSC of the catalyst, not the emissions. The accuracy of the monitoring system is improved by having a good mathematical model for OSC.
The system uses actual emission data stored and sampled in appropriate histograms and determines the vehicle""s potential for meeting regulatory emission standards by comparing measurements to a threshold value. The emission data is determined from the development process.
This is achieved in an on-board vehicular catalytic converter monitoring system (method and/or apparatus) for monitoring the Oxygen Storage Capacity of the catalyst converter""s and generating signals during vehicle operation.
The vehicle""s electronic control module simultaneously processes the signals to determine when and if the signals have exceeded regulatory standards.
Accordingly, the system provides a vehicle on-board emissions monitoring system which utilizes actual emission data produced by the specific monitored vehicle (threshold value) to construct relatively large data bases coinciding with government mandated FTPs which can be statistically validated to insure accurate and reliable compliance with vehicular emission standards.
The system provides an accurate vehicular on-board automotive emission monitoring system which produces consistent, reliable readings indicative of the gaseous pollutants actually emitted by the vehicle.
The system also provides an automotive on-board emission monitoring system which can be varied to determine compliance with any emission test protocol and which can automatically adjust for aging of the catalytic converter.
The system provides an on-board vehicular emission monitoring system which is simple and inexpensive.
In an embodiment, the system provides a vehicular emission monitoring system which utilizes the vehicle""s existing ECM to monitor the emissions produced by the vehicle.
Still another object of an exemplary embodiment of the present invention is to provide an on-board vehicular emission monitoring system which is predictive of an expected failure of the vehicle""s emission system so that the vehicle can be scheduled for preventive maintenance.
Still another object of an exemplary embodiment of the present invention is to provide an on-board emission diagnostics system which after determining that a failure of the emissions system by a relatively fast processing routine then proceeds to execute further processing routine to pinpoint the components in the system which caused the failure.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the Detailed Description of the Invention set forth below taken in conjunction with the drawings.